Since listening-adapted digital coding methods have been standardized, they have been used to an increasing extent. Examples for such cases of use are the digital compact cassette, the minidisk, digital terrestrial radio broadcasting and the digital video disk. When coding is effected by means of listening-adapted coding methods, artificial products or artifacts may, however, occur, which did not occur in analog audio signal processing.
For judging or evaluating a specific encoder, listening test with test persons were carried out in the past. Although the average result provided by such listening tests is comparatively reliable, there is still a subjective component. Furthermore, listening tests executed with a certain number of test persons are comparatively complicated and therefore comparatively expensive. Hence, measurement methods have been developed for a listening-adapted evaluation of audio signals.
Such a measurement method is described e.g. in DE 196 47 399 C1. The method of listening-adapted quality evaluation described in this publication models all non-linear hearing effects onto a reference signal as well as onto a test signal. The listening-adapted quality evaluation is carried out by means of a comparison in the cochlear domain. In so doing, the excitations caused in the ear by the test signal and by the reference signal are compared. For this purpose, both the audio reference signal and the audio test signal are divided into their spectral components by a filter bank. By means of a large number of filters whose frequencies overlap, a sufficient resolution with respect to time as well as frequency is guaranteed. Hence, a mono audio test signal, which is derived from an audio reference signal by coding and subsequent decoding, can be evaluated with regard to its quality.
The measurement method described in DE 196 47 399 D1 also permits an evaluation of the quality of stereo signals, i.e. two-channel signals. For this purpose, non-linear preprocessing is carried out with the left and with the right channel of the audio test signal and of the audio reference signal; this preprocessing emphasizes transients in a frequency-selective manner and reduces stationary signals. In particular, different detections of the error probability are carried out with the left channel of the audio reference signal and with the left channel of the audio test signal as input signals, with the right channel of audio reference signal and with the right channel of the audio test signal as input signals, with the left channel of the preprocessed audio reference signal and with the left channel of the preprocessed audio test signal as input signals and with the right channel of the preprocessed audio reference signal and with the right channel of the preprocessed audio test signal as input signals so as to obtain a measure of the quality of the stereophonic audio test signal.
A disadvantage of the known method for listening-adapted quality evaluation of audio signals is the fact that the stereo ability is limited to a reproduction by headphones alone. In other words, the audio test signal which enters the ear of a listener is compared with the audio reference signal which enters the ear of a listener. This means that effects produced by a room, such as reflections on the walls, on the ceiling and on the floor, multiple reflections, attenuations, etc., are not taken into account. Furthermore, known quality-evaluating methods are not able to take into account any directional characteristic of the human ear, i.e. it makes no difference whether a signal comes from the rear, from the front or from the side. Known measurement methods are only applicable to headphone reproduction in the case which the acoustic signal is emitted by the headphone loudspeaker, which is normally arranged directly on the ear, and is introduced in the ear or the quality-evaluating process.
The known method is also disadvantageous insofar as a listening-adapted quality evaluation of multi-channel signals, such as e.g. 5-channel signals, which become more and more common and which are known under the headword “Dolby surround”, has been absolutely impossible up to now.